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Monday, March 15, 2010

The Frog Spawn Picture

Raise your hand if you've seen this picture before:

It's an illustration of a heavy ion collision among two lead nuclei at a beam energy of 160 GeV per nucleon. I've seen this picture over and over again in talks, leaflets, and even printed in books, typically as a motivation for why heavy ion physics is the thing to do and the quark gluon plasma is cool. Followed by a praise of whatever model it is that the speaker/author is working on.

Now raise your hand if you know who made the picture and how it was made?

Nobody? Well, there's the rub. As often as I've seen the picture, as often the credits were missing. If it is credited to anybody, it's credited to a CERN press release of February 2000 that was summing up the results of the CERN-SPS heavy-ion program before the start of RHIC at Brookhaven. The picture was used as an illustration for that.

As it happens, Stefan and I know exactly who spent weeks on this figure and how its production came along, since at that time we were sharing an office with the unknown, uncredited physicist behind it. The picture was made by Henning Weber, then a PhD candidate at the Institute for Theoretical Physics in Frankfurt. Henning has meanwhile left academia, which is why you've probably never heard of him. The picture is a visualization of data generated with a numerical model called UrQMD (Ultra-relativistic Quantum Molecular Dynamics). From the code's numerical output that's usually just a looong list of numbers, Henning made a couple of videos showing how a heavy ion collision in this model looks like. He still has his website up, you can look at the videos here.

The grey balls are color-neutral hadrons which the initial nuclei consist of. Shown is a not-quite central collision, one in which the nuclei have a non-zero impact parameter. The red, blue and green balls represent temporarily unconfined quarks in the collision region. After the collision, the quarks hadronize again. Well, actually, the UrQMD code does not explicitly treat color degrees of freedom, so the colors are an artistic rendition of what technically are called "preformed hadrons". The below shows a screenshot of one of Henning's movies. Same picture, but with modest credits to the Frankfurt UrQMD group at the bottom:

So what happened? Well, Henning was asked by his supervisor to provide a visually appealing picture for an upcoming CERN press conference. Henning sat down and spent some days and nights on the picture that he would later refer to as the "frog spawn picture," because said supervisor insisted on making the balls semi-transparent giving them the appearance of fish eggs. This association was even stronger after the relativistic squeeze of the nuclei was removed. More accurately, the nuclei should be flattened to about 1/10 of the initial size in the direction of motion – the Lorentz gamma factor for the SPS fixed-target collisions at 160 GeV/nucleon is, in the center-of-mass frame, γ = 9.2.

The picture then was sent to CERN and used in the press release. And somewhere along the line the bottom with the credits vanished.

34 comments:

A couple of things that you can do if you want to help Henning get search-engine credit:

[1] Edit the blog page source code so that the image has an alt="" field, and fill in a long title explaining what the image is and who originated it. If you want to cover all the bases, give it a title="" field as well. That way, Google will associate that text with the image, and (by association) also with the other similar copies of the image found on the web.

[2] Get hold of Picasa or some other image-tagging software, and do the same thing with the file's internal embedded extended .JPG title field, and then also fill out a series of embedded keyword tags for the image, making one of them Hanning's name. Then re-upload.

That way, if anyone downloads the picture off your blog to re-use it, although the HTML tags are left behind, the infomation's all still embedded in the file, and moves with it.

I suspected that it was a computer simulation, but I'm not sure how accurate it is (can we know?) since as you pointed out one nucleus is traveling "massively" faster than the other. The quarks in the picture represent the central quarks and not the virtual quark-anti-quark zoo surrounding the protons and neutrons, yes? I think yes, otherwise the picture would be even busier. And of course the quarks aren't actually colored, but it looks pretty.

Well thanks to you two Henning Weber gets at least recognition here, although that’s the typical plight of graduate students; lots of work and little recognition. One thing though he’s aware who the creator is and credits or no credits that can’t be taken away. I’m sorry to learn he’s left academia though and do hope he’s doing well in whatever it may be.

It is interesting you referred to it as frog spawn as that’s exactly what it looks like if left uncoloured and has me mindful of where I lived as a child there was a small pond nearby where we would go and collect some in a jar to watch the changes in the eggs to eventually see the pollywogs appear. I don’t see any here though so that’s where the semblance ends and so we can’t say quarks then turn to frogs. Never the less it still can be noted as a leap forward in our understanding of nature so we could say it does share an action:-)

It's irrelevant whether one nuclei is traveling faster than the other or not. You go into the center of mass frame, then they both move with the same velocity in opposite directions (that's the frame used for the visualization). How accurate is the simulation? Well, UrQMD is one of a handful of numerical models that are being used to simulate heavy ion collisions in the hope to understand essential features of the nuclei. I haven't had much contact to the community for several years, so I'm not the most reliable source, but my impression was roughly that every code has its advantages and disadvantages with UrQMD doing one of the better jobs. To be concrete that means how well can you fit the actual data. Typically, you'll see a plot with the data on it and with the results of several models, that fit the data better or worse. Stefan can probably tell you more details since he's been working with one of these simulations for his thesis. Best,

If you pipe this and this into Google translate you might get an impression what Henning is up to now.

We also had a pond nearby where I grow up, and we did collect frog spawn too. Today I sometimes go for a walk there and see the kids trying to catch a frog like we did back then. I guess there's things that never change :-) Best,

Thank you Bee. I knew the velocity vectors would be the same, but I forgot I didn't know the exact speed they were traveling at, and therefore whether relativistic length contraction and mass increase (inertia, I know) would be significant, then I realized that was a bad thought because then again: relativity. Wow, I realized I have to drag my Intro. to Physics Vol. 2 text (by Griffiths I think it is) out of the attic. I haven't had those thoughts in 31 years. The Internet is nice but nothing beats a dedicated textbook. First time I read it because I had to; this time will be for fun. Extra bonus: it's from the late 70's so no Strings Theory.

Hi Bee,What I think ErkDemon means is find a photoeditor which can edit EXIF data, e.,g., like this that shows up on your photograph of PI construction:

Camera Maker: Canon

Camera Model: Canon DIGITAL IXUS 60

Image Date: 2009:03:17 21:33:20

Focal Length: 5.8mm

Aperture: f/5.6

Exposure Time: 0.0020 s (1/500)

Exposure Bias: none

Metering Mode: Matrix

White Balance: Auto

Flash Fired: No (Auto)

Color Space: sRGB

Photographer: Sabine

Picasa is Google's free photography software. In particular it contains rather impressive face recognition algorithms. So it can catalog your photo collection by the people it contains. Be careful if you use it to tell it not to send photo info to Google (if that is what you want).

Yeah, but not the kids. One day I suspect they'll hunt for virtual frogs in a virtual pond. I can just picture it, the great catch-the-frog game, 5 years and up, requirements: Windows XP and up, 120 MB disk space. Best,

The snapshot shows data for the positions of particles from a simulation of a collision of lead nuclei at the CERN SPS, just 1.6 fm/c after the first contact between the nuclei (that is, after the time it takes light to travel 1.6 femtometer, or 5×10^−24 seconds).

In this experiment, a beam of fully ionized lead ions is shot on a fixed lead target. The energy of the lead nuclei in the beam is 160 GeV per nucleon, or 33 TeV per ion. At this energy, the velocity of the ions in the beam is 99.998 per cent the speed of light.

The simulation is run in the center-of-mass frame, and the results boosted to the laboratory frame to compare with experimental data. In the center-of-mass-frame, the velocities of the colliding nuclei are 99.4 per cent of the speed of light each, meaning a Lorentz contraction by a factor 9.2 (the γ factor mentioned in the post).

As you can guess, the code has to use relativistic kinematics throughout.

The quarks in the picture represent the central quarks and not the virtual quark-anti-quark zoo.

That's correct. Virtual sea quarks and gluons are not shown, just valence quarks. Virtual stuff would clutter the image even more.

But as mentioned in the post, the UrQMD code that was used to create the figure actually doesn't have quark degrees of freedom - it always deals with hadrons and hadronic resonances (protons, neutron, pions, kaons, lambdas, sigmas, deltas... the complete baryon and meson multiplets are included) which can be produced, can scatter and decay and in the very early stages of the collisions are used as "placeholders" for the actually deconfined quarks.

These "placeholder" hadrons are represented by the coloured balls, which are supposed to show the valence quarks.

I'm not sure how accurate it is (can we know?)

That's of course the one-million dollar question in all these models - the issue is that one usually cannot measure directly the actual dynamical variables at the early stage of the collisions, but just the energy and momenta of the final hadrons which reach the detector. Everything else has to be inferred from these data. So, essentially the code has to reproduce these observables (which UrQMD does), and then we can be quite confident that the simulation is not completely off in the unobserved early stage.

Depends on what you mean with "determine." The model itself tells you exactly which proton did exactly what. As Stefan said above, the one-million dollar question is of course how well is that a description of reality. The only thing you can do is compare the results of the simulation with the actual data. The better the match, the more confidence you have that the model is an accurate description of reality. Best,

So you also as a child collected frog eggs in a jar of water to be able to watch the changes that would then reveal themselves. It’s strange as I always had you pegged as a city kid, where such things are simply not there to observe. I can remember I marvelled at how could it be possible for something which at first so seemingly simple changed to become so complex, not releasing that within was a complexity that still remains not completely understood. I’ve always felt sorry for children that will never have opportunity to be exposed to this sense of wonder, yet more and more confined to the images provided, as to imagine them to be those prisoners Plato described so long ago.

Well it looks like Henning has gone from one extreme to the other, from attempting to model as to confirm the predicted actions of something never seen, to developing models to better predict what although plainly seen, so complex in action still resists us knowing what it will do beyond a short time. So as these frog eggs we wondered about in our childhood, I still find there is more that connects things then not and all that’s required first able to pay attention long enough to haveus to wonder,

” Physicists like to think that all you have to do is to say, here are the conditions , now what happens next?”

-Richard Feynman ,The Feynman Lectures , volume three

”For want of a nail, the shoe was lost;For the want of a shoe, the horse was lost;For the want of a horse,the rider was lost;For the want of a rider, the battle was lost;For want of a battle,the Kingdom was lost!”

I've grown up in a suburb of Frankfurt, not in the city. Besides that, people who haven't been there before tend to be surprised how green Germany is (can't recall if you've visited central Europe?). I mean green as in green, not in environmentally conscious. Look at this map. See the green spot in the middle? That's why Europeans are concerned about climate change. Just by looking at the map it looks like a lucky spot.

You're right of course in that complexity is a topic that will likely remain subject of research for many decades. It's a big step though from heavy ion collisions to frogs. On that complexity scale, heavy ions are actually dramatically simple objects. And see how we're struggling to understand them already. It's interesting actually how much open questions there are in the field given that we know the underlying fundamental theory. So much about reductionism. Best,

And so as it happens there is more that connects things than not, as like you I grew up in a suburb near Halifax, with my house boarding on the edge of a forest which was much the same as what you describe. So I find that although complexity has aspects that confound prediction, it still has what is called the strange attractors, that despite everything else defines what can be from what never can. The question of course being are the attractors simply phenomena themselves or rather what they serve to limit as such?

”What else, when Chaos draws all forces inward to shape a single leaf”

-Conrad Aiken (as quoted by Geick to introduce a chapter in his book Chaos)

I grew up in a suburb near Halifax, with my house boarding on the edge of a forest which was much the same as what you describe.

Did you ever go whale-watching out of Halifax and see a humpback whale (about 40 ft long) do his/her breaching of the surface 30 times or so? I did in Cape Cod Bay once, the only time I ever went whale-watching. I was told the odds of seeing that on any one trip are small.

Stefan, thanks for the details. A wonderful picture, much to ponder, and much to teach and learn.

Looks like those two ions "winged" each other, eh? Because I'm guessing the velocity vectors line is left to right. True?

Also, the virtual stuff struck me as crazy the first time I learned of it.

I recommend you watch the video I referred to in the post, it will clarify your question about "what happened." As I wrote in the post, it's a not-quite central collision, meaning the trajectories of the ions' centers-of-mass are slightly offset to each other. I suppose that's what you meant with "winged" each other. Best,

There's videos?! Wow! That's what I get for skim reading! Missed it! I was just looking at the picture.

Now that I'm seen the vids, Wow!, and thanks. I like the way hadrons pop up right away at t>picture above t. Guarks don't like being unconfined (or looosly defined therfore undecided which other quarks they'll marry to be happy hadrons) for long, do they? Great stuff. Superb teaching tool.

I sort of wish the uncollided bits of the ions would spin a little, a little bowling "English", a little angular momentum. But given this time-span is very short, I guess not.

Sorry about an request for an off topic discussion.I don't know where to ask the questions raised in my mind by the picture.

Can the instrument determine what happened to the protons that did not participate in the making of the perfect liquid and the emission of jets?

Bee said...Hi Jal,

Depends on what you mean with "determine." The model itself tells you exactly which proton did exactly what. As Stefan said above, the one-million dollar question is of course how well is that a description of reality. The only thing you can do is compare the results of the simulation with the actual data. The better the match, the more confidence you have that the model is an accurate description of reality. Best,

Hummm!

My question is a result of reading the following paper.

Do I assume that he protons just keep going round and round until they "crash" and then they are "dumped"?

What is meant by 'dumped" and and etc.?There is a lot of energy that need to be absorbed.

http://arxiv.org/abs/0911.5430 First proton--proton collisions at the LHC as observed with the ALICE detector: measurement of the charged particle pseudorapidity density at sqrt(s) = 900 GeV

Yes I’ve seen whales many times, yet never aboard a boat chartered solely for the purpose. If you know where to go one can simply view them from the land in many places throughout the province. The longest and closest I’ve observed them was as a result of taking the family when my girls were young back there on a driving vacation. We travelled stateside to Bar Harbor Maine to drive then onto an ocean ferry for a six hour voyage to Yarmouth N.S., as this is the closest landing point. After being an hour out several whales travelled beside us for over two hours. However in this case it would be hard to say who was watching who :-)

Oh yes, you can get there entirely by land and without the need to go through the U.S, yet I wanted to broaden their cultural experience and to give them a better feel for the place, with travelling there in part by sea. If you’ve never been there I would recommend it and boast that while not the old world, the place,people and culture are unique in terms of the New World. I apologize to Bee and Stefan for straying of topic, yet with your question and the talk of our youth I just for an instant was feeling a little home sick:-)

Hi Bee, how can I start a guest blog?Oh a first question and interest! Neuro-quantology is the attempt to link between neuroscience and quantum mechanics. I am critical about this as it could give the impression of touching esoterism. But having in mind the statical properties of neuronal processing and network properties, quantum processes might be involved at critical bifurcation points .

What you have left out of the discussion is that while this may be a pretty picture, it isn't what they have found in the experiments. From what I have read about RHIC, the exeriments have failed to find the evidence they were looking for a quark-gluon plasma. What they found instead was a liquid that defies every prediction of the standard model.

RHIC was build for the purpose of finding that quark-gluon plasma and it basically failed to do that. This is ,once again, a massive failure to find individual quarks even at the energies in which they were supposed to appear. I hear the sound of a billion dollars being flushed down the drain at Brookhaven.

You should show a picture of what was really found and try to explain that. I'm guessing that there is no quark-gluon plasma, but rather what we are seeing is a nothing more than a cloud of loosely bound positrons and electrons.

fhblogger: The picture stems from the pre-RHIC era and it doesn't actually contain details about the properties of the plasma. For more on the qgp, I would recommend you read the above mentioned post, which also contains some nice pictures.